Field of the Invention
The invention concerns a method for recording magnetic resonance data of a target region of an examination object, in particular of a patient, with a magnetic resonance scanner, of the type wherein the recording process is divided into a number of successive subsections that occur after a repetition time in each case, and wherein, before each recording of magnetic resonance data of a subsection with a measurement sequence, an adiabatic preparatory pulse is radiated that inverts the longitudinal magnetization of a saturation molecule type, from which no magnetic resonance data are to be recorded, and at least one excitation pulse of the measurement sequence is thereafter radiated, spaced from the preparatory pulse by an inversion time. The invention also concerns a magnetic resonance apparatus, and a non-transitory, electronically readable data storage medium that implement such a method.
Description of the Prior Art
Saturation methods, in particular for fat saturation, are known within the framework of magnetic resonance imaging. In particular “spectral attenuated inversion recovery” (SPAIR) has also been proposed, which is based on the inversion of the longitudinal magnetization within a frequency band of the magnetic resonance spectrum. An adiabatic preparatory pulse inverting the longitudinal magnetization is omitted. The excitation pulse of the actual measurement sequence should then ideally be radiated at a time after a so-called inversion time, such that the longitudinal magnetization of the saturation molecule type, of which the magnetic resonance signal is to be suppressed, is null where possible. As mentioned, fat saturation is a frequent application for the SPAIR technique.
If the preparatory pulse (frequently also referred to as the inversion pulse) is repeated multiple times, for example during multislice imaging or segmented k-space sampling, there is usually a repetition time between two preparatory pulses. If this repetition time is in the range of the longitudinal relaxation time T1 of the spectrally inverted magnetization, the optimum inversion time TI initially continues to change as the measuring time progresses, since the first preparatory pulse is based on the maximum possible, i.e. completely relaxed, longitudinal maximum magnetization, but because of the complete relaxation not yet being produced after the expiration of the repetition time, the optimum inversion time T1 will no longer be reached. This means that, for a constant choice of the inversion time for all repetitions, the magnetic resonance signal of the saturation molecule type, i.e. in the inverted frequency band, will oscillate in the first repetitions, until a steady state is achieved. Thus a signal inconsistency is present between the different repetitions. In the case of fat saturation, this effect will result in image artifacts or fat signal inhomogeneities in the magnetic resonance image data set.
In order to solve this problem, it has been proposed to emit a spectrally selective preparation pulse be output before the first time that the preparatory pulse is emitted, which partly saturates the longitudinal magnetization in the frequency band of interest, i.e. for the saturation molecule type, and allows the steady state to be reached without multiple repetitions. In this case the magnetic resonance signal that is recorded in the first repetitions can be used for the imaging, wherein artifacts are greatly reduced. Such a method is described, for example, in DE 10 2014 204 995 A1.
The use of a spectrally selective preparation pulse also has disadvantages, however, because the spectral effectivity of the preparation pulse does not correspond to that of the preparatory pulse, and the preparation pulse is not adiabatic. For these reasons restrictions of the spectral and spatial effectiveness of the preparatory pulse occur when the steady state is being reached.